Technical Field
[0001] The present invention relates to wireless communications, and more particularly,
to a method and apparatus for a multiple input multiple output (MIMO) operation in
a multi-cell wireless network in which a plurality of base stations participate in
cooperative communication.
Background Art
[0002] Recently, with the generalization of information communication services, the introduction
of various multimedia services, and the advent of high-quality services, a demand
on wireless communication services has been rapidly increased. Various multiple antenna
systems are proposed to provide high-speed, high-quality data services in wireless
communication. In the multiple antenna system, a transmitter and a receiver use multiple
antennas. The multiple antenna system is also referred to as a multiple input multiple
output (MIMO) system.
[0003] One example of the MIMO system is a multi-user (MU) MIMO system for allowing multiple
users to effectively use spatial resources. Many techniques have been proposed to
increase channel capacity in the MU MIMO system, and one of them is a dirty paper
coding (DPC) scheme. According to the DPC scheme, data of other users is removed in
advance by a base station (BS) when the data acts as interference to each user, and
thus interference from other users is reduced. Therefore, the MIMO system can provide
maximum channel capacity. However, the DPC scheme has a problem in that it is difficult
to be implemented in an actual MIMO system since not only a transmitting end requires
much channel information but also computational complexity is high.
[0004] One example of a method for increasing channel capacity of the MU MIMO system is
a per user unitary and rate control (PU
2RC) scheme in which spatial resources are allocated simultaneously to different users.
The PU
2RC scheme is proposed as a contribution document entitled "
Downlink MIMO for EUTRA" of 3GPP TSG RAN WG1 #44/R1-060335 by Samsung Electronics
Ltd. According to this scheme, each user selects a precoding matrix capable of maximizing
a channel transfer rate of each user from a plurality of precoding matrices having
an orthogonal basis, and feeds back a signal to interference plus noise ratio (SINR)
and an index of the selected precoding matrix to a BS. On the basis of information
fed back from each user, the BS determines and transmits a user and a precoding matrix
for maximizing a total data rate.
[0005] The PU
2RC scheme uses a spatial multi-user diversity to increase system capacity, and has
an advantage in that it can be implemented in an actual MIMO system due to low computational
complexity. However, the PU
2RC scheme determines the user and the precoding matrix by considering only a single-cell
environment and does not consider a multi-cell environment where inter-cell interference
occurs. In particular, since a user located in a cell edge experiences a lot of interferences
from neighboring cells, it is difficult to ensure a data rate of a specific level
or higher when inter-cell interference is not considered.
[0006] Several schemes have currently been discussed to solve the problem of inter-cell
interference occurring in the multi-cell environment. One example of these schemes
is a BS cooperation scheme. A MIMO operation technique based on cooperation between
BSs has recently been proposed in many papers. For example, a paper entitled "Base
Station Cooperation for Multiuser MIMO : Joint Transmission and BS Selection" by HongyuanZhang,
HuaiyuDai, and QuanZhou describes a DPC scheme based on BS cooperation or joint zero
forcing beam forming (ZF-BF) scheme. According to this paper, all BSs have to perfectly
know channel information in order to apply the DPC scheme based on BS cooperation
or the joint ZF-BF scheme. Therefore, it is almost impossible to implement the BS
cooperation scheme in an actual system due to feedback or complexity.
[0007] The BS cooperation scheme is also discussed in a paper entitled "Receiver-Enhanced
Cooperative Spatial Multiplexing with Hybrid Channel Knowledge" by Hilde Skjevling,
David Gesbert, and Are HjØrungnes. In this paper, a method for BS cooperation is proposed
under the assumption that channel information of a neighboring cell uses only statistic
information, and channel information of a cell for receiving a service is perfectly
known. However, since the channel information of the cell for receiving the service
of the BS is assumed to be perfectly known, it is also difficult to apply the scheme
proposed in this paper to an actual system.
U.S. patent application publication No. 2007/248172 A1 describes a system and method for transmitting signals in cooperative base station
multi-user MIMO networks.
Disclosure of Invention
Technical Problem
[0008] The present invention provides a method of configuring a precoding matrix for a cooperative
multiple input multiple output (MIMO) operation which considers a multi-cell environment
where inter-cell interference may occur and which can be implemented in an actual
wireless communication system, and a transmission method using the precoding matrix.
[0009] The present invention also provides a method of exchanging information between base
stations (BSs) for a cooperative MIMO operation which considers a multi-cell environment
where inter-cell interference may occur and which can be implemented in an actual
wireless communication system.
[0010] The present invention also provides a user scheduling method for a cooperative MIMO
operation which considers a multi-cell environment where inter-cell interference may
occur and which can be implemented in an actual wireless communication system.
Solution to Problem
[0011] In an aspect, a method of configuring a multi-cell precoding matrix for a multiple
input multiple output (MIMO) operation in a wireless communication system in which
a plurality of base stations participate in cooperative communication is provided
as set forth in the appended claims.
Advantageous Effects of Invention
[0012] An embodiment of the present invention considers a multi-cell environment where inter-cell
interference may occur unlike in the conventional per user unitary and rate control
(PU
2RC) considering only a single-cell environment. Therefore, a multiple input multiple
output (MIMO) operation can be more accurately implemented in an actual wireless communication
system. According to the embodiment of the present invention, it is not necessary
for all base stations to perfectly know channel information. In addition, a feedback
overhead is not high, and complexity is low. Therefore, the MIMO operation can be
implemented more properly in an actual wireless communication system.
Brief Description of Drawings
[0013]
FIG. 1 is a diagram showing an example of a multi-cell precoding matrix according
to an embodiment of the present invention for a multiple input multiple output (MIMO)
operation in a wireless communication system in which two base stations cooperate
with each other.
FIG. 2 is a flowchart showing an example of a feedback method for a MIMO operation
in a multi-cell base station cooperative wireless communication system.
FIG. 3 is a diagram showing a user scheduling scheme through information exchange
for a MIMO operation in a two-cell base station cooperative communication system.
FIG. 4 is a flowchart showing an example of an information exchange procedure between
base stations for a MIMO operation in a multi-cell base station cooperative wireless
communication system.
FIG. 5 is a block diagram showing a wireless communication system to implement an
embodiment of the present invention.
Mode for the Invention
[0014] Hereinafter, embodiments of the present invention will be described in detail with
reference to the accompany drawings. A cooperation scheme of two cells in a multi-cell
environment will be described in the embodiments described below for exemplary purposes
only, and thus the present invention is not limited thereto. The same content described
with respect to the cooperation scheme of the two cells can also be applied without
much change even in a case where the number of base stations (BSs) participating in
cooperation increases. Matters to be considered when the number of BSs increases will
be additionally described below.
[0015] First, a method of configuring a precoding matrix for a cooperative multiple input
multiple output (MIMO) operation will be described according to an embodiment of the
present invention.
[0016] Each BS has N
T transmit (Tx) antennas. A receiving end (e.g., a user equipment) has N
R receive (Rx) antennas. When L(≤N
T) pieces of data are transmitted through the Tx antennas, Rx signals in a single-cell
environment, not in a multi-cell environment, can be expressed by Equation 1.
[0017] Herein, Y
k,i denotes an N
R×1 Rx signal vector of a k
th user of an i
th cell, H
k,i denotes an N
R×N
T MIMO channel between the k
th user and an i
th BS, W
i denotes an N
T×L unitary matrix (i.e., a precoding matrix) of the i
th BS, N
k denotes an N
R×1 noise vector, and S
i denotes an L×1 Tx symbol vector of the i
th BS.
[0018] A communication system in which two cells cooperate with each other may be regarded
as a single-cell system which has 2×N
T Tx antennas and which transmits 2xL pieces of data. Therefore, a precoding matrix
for a two-cell cooperative system has to be a 2N
T×2L unitary matrix (hereinafter, a precoding matrix used in such a cell cooperative
system is referred to as a 'multi-cell precoding matrix'), and it is not allowed to
directly use an N
T×L precoding matrix used in the conventional single-cell system (hereinafter, referred
to as a 'single-cell precoding matrix').
[0019] According to the precoding matrix compliant with the embodiment of the present invention,
that is, according to a method of configuring a multi-cell precoding matrix, the multi-cell
precoding matrix is created by using the conventional single-cell precoding matrix.
For example, a multi-cell precoding matrix for a two-cell cooperative system can be
used by using Equation 2.
[0020] Herein, W
co denotes a multi-cell precoding matrix, and W
p and W
q respective denote a p
th single-cell precoding matrix and a q
th single-cell precoding matrix in a single-cell precoding matrix set (i.e., a codebook).
According to the embodiment of the present invention, p and q may have the same value.
In addition, any combination for allowing α
11,α
12,α
21,α
22 to satisfy
(where I is a unitary matrix) is possible. Herein, W
coH is a a Hermitian matrix of W
co. According to Equation 2, a desired number of 2NTx2L unitary matrices (i.e., multi-cell
precoding matrices) for BS cooperation can be created by using a single-cell precoding
matrix selected by properly combining p and q and by determining α
11,α
12,α
21,α
22 each of which satisfies
[0021] For example, assume that the number G of single-cell precoding matrices is 4. In
this case, p and q may have any value in the range of 1 to 4. According to the embodiment
of the present invention, the number of multi-cell precoding matrices that can be
created may be any number less than 16, i.e., a combination of p and q. For the 4
multi-cell precoding matrices, the combination of p and q can be determined to be
p=q=1, p=q=2, p=q=3, p=q=4 . In addition, each of α
11,α
12,α
21,α
22 satisfying
may be α
mn = exp(j2π(m-1)(n-1)/2) (where m and n is 1 or 2). Equation 3 shows examples of a
multi-cell precoding matrix that can be created by using Equation 2.
[0022] A multi-cell precoding matrix W
co is obtained using Equation 2 and includes precoding matrices respectively used by
two BSs cooperating with each other. For example, a precoding matrix used by a BS
1 may be an upper N
T×2L matrix in any one of the multi-cell precoding matrix W
co, and a precoding matrix used by a BS 2 may be a lower N
T×2L matrix of the multi-cell precoding matrix W
co. In FIG. 1, a g
th multi-cell precoding matrix is indicated by W
co,g and precoding matrices respectively used by two cooperative BSs (i.e., the BS 1 and
the BS 2) are indicated by W
co,g,1 and W
co,g,2.
[0023] Next, a feedback method for a MIMO operation will be described with reference to
FIG. 2. The feedback method described below uses a multi-cell precoding matrix obtained
according to the aforementioned embodiment of the present invention. FIG. 2 is a flowchart
showing a feedback method for a cooperative MIMO operation in a multi-cell wireless
network according to an embodiment of the present invention.
[0024] Referring to FIG. 2, a receiving end obtains signal to interference plus noise ratios
(SINRs) for all pieces of Tx data (i.e., 2L pieces of data) or for all Tx antennas
(i.e., 2N
T antennas) (step S11). The receiving end may be a user equipment. The SINR is obtained
using an Rx signal in the receiving end under the assumption that power is equally
allocated to antennas transmitting data (i.e., 2L antennas) or all Tx antennas (i.e.,
2N
T antennas).
[0025] When two BSs participate in cooperative communication by using the multi-cell precoding
matrix obtained according to the aforementioned embodiment of the present invention,
the Rx signal can be expressed by Equation 4.
[0026] Herein, S
co denotes a Tx signal vector having a size of 2L×1, W
co,g,i denotes a g
th multi-cell precoding matrix used by an i
th BS, and H
eff,g denotes an effective channel when using the g
th multi-cell precoding matrix.
[0027] A detailed method of calculating the SINR may differ depending on a type of a receiver
used in the user equipment. The receiver may be a zero-forcing (ZF) receiver or a
minimum mean squared error (MMSE) receiver. For the ZF receiver, an SINR of an i
th stream for a g
th multi-cell precoding matrix of a k
th user can be expressed by Equation 5. For the MMSE receiver, the SINR of the i
th stream for the g
th multi-cell precoding matrix of the k
th user can be expressed by Equation 6.
[0028] Herein, p denotes a signal to noise ratio (SNR), I
2L denotes a 2Lx2L unitary matrix, H
eff,g denotes an effective channel when using the g
th multi-cell precoding matrix, and [·]
i,i denotes a value of an i
th diagonal element.
[0029] If the number of Rx antennas is 1, an SINR of an i
th stream for a g
th multi-cell precoding matrix of a k
th user of a 1
st cell can be expressed by Equation 7. On the other hand, in case of using the conventional
PU
2RC, the SINR of the i
th stream for the g
th multi-cell precoding matrix of the k
th user of the 1
st cell is expressed by Equation 8 when the number of Rx antennas is 1.
[0030] In Equations 7 and 8, W(i) denotes an i
th column of a precoding matrix W. The second term in the denominator of Equation 8
denotes interference of a neighboring cell and acts as a factor of decreasing an SINR
(herein, the SINR of Equation 8 expresses an SINR of a user in the 1
st cell). On the other hand, in Equation 7, the second term appears both in the denominator
and the numerator since it is an SINR for a case of BS cooperation.
[0031] Each user (or user equipment) selects a precoding vector having an optimal SINR among
SINRs obtained in step S11 (step S12). The precoding vector having the optimal SINR
may be one precoding vector capable of maximizing a data rate among 2L spatial precoding
vectors under the assumption that power is equally allocated.
[0032] For the precoding vector having the optimal SINR and obtained in step S12, the user
equipment transmits to the BS an SINR value, indication information of a precoding
vector, and indication information of a precoding matrix (step S13). In case of BS
cooperation as described in the embodiment of the present invention, each user equipment
can receive a data stream corresponding to a smaller value between N
R and 2L, and the smaller value will be expressed hereinafter by 'mim(N
R, 2L)'. In step S13, each user equipment transmits index information regarding an
SINR value and a precoding vector to the BS according to three methods described below.
The index information regarding the precoding vector is an example of indication information
regarding a selected precoding matrix. The three methods described below are for exemplary
purposes only, and thus the embodiment of the present invention is not limited thereto.
It is assumed herein that the number of multi-cell precoding matrices is G.
[0033] In the first method, each user equipment transmits to a BS a total of mim(NR, 2L)
higher SINRs for the G multi-cell precoding matrices and precoding vector indices
corresponding to the SINRs. This method is suitable when a feedback link is sufficient
since much information is fed back from the user equipment. In this method, a spatial
multiplexing (SM) gain and a spatial multi-user diversity (MU) gain can be obtained.
[0034] In the second method, for one multi-cell precoding matrix capable of maximizing a
data rate among the G multi-cell precoding matrices, each user equipment transmits
to a BS a total of mim(N
R, 2L) higher SINRs and precoding vector indices corresponding to the SINRs. In this
method, an SM gain and a spatial MU gain can also be obtained.
[0035] In the third method, for one multi-cell precoding matrix capable of maximizing a
data rate among the G multi-cell precoding matrices, each user equipment transmits
to a BS one SINR for maximizing a data rate and a precoding vector index corresponding
to the SINR. In this method, only a spatial MU gain can be obtained.
[0036] For the aforementioned three methods, an amount of data (i.e., a feedback amount)
transmitted to the BS for each user equipment can be summarized as described in Table
1 below.
Table 1
[0037]
[Table 1]
|
|
First method |
Second method |
Third method |
feedback amount |
SINR |
G·min(NR, 2L) |
min(NR, 2L) |
1 |
Bits |
G·min(NR, 2L)· log2(2L) |
min(NR, 2L)· [log2(G)+log2(2L)] |
log2(G)+ log2(2L) |
gain |
SM, MU |
SM, MU |
MU |
[0038] Referring continuously to FIG. 2, the BS selects a multi-cell precoding matrix for
maximizing a total data rate by integrating information fed back in step S13 (step
S14). The selected multi-cell precoding matrix is used in user scheduling by which
the BS assigns users to 2L precoding vectors of a corresponding matrix.
[0039] A set of precoding vectors fed back from the users to the BS in the aforementioned
step S13 may be insufficient to select one multi-cell precoding matrix by the BS in
step S14. This is a case where the number of users is insufficient or only some precoding
vectors are fed back. For example, assume that 4 precoding vectors exist, and all
users perform feedback by selecting only a 1
st vector or a 2
nd vector. In this case, the users cannot be assigned to a 3
rd vector and a 4
th vector. According to the embodiment of the present invention, the following methods
can be used in this case.
[0040] In the first method, users are not assigned to precoding vectors for which feedback
is not required. The following procedure is performed in this case. First, the BS
determines adaptive modulation and coding (AMC) by using an SINR which is first fed
back. Then, the BS reports a scheduled precoding vector to a corresponding user, and
determines again AMC by receiving a feedback of an SINR depending on the precoding
vector.
[0041] In the second method, the users are randomly assigned to the precoding vectors for
which feedback is not required. In this case, an additionally assigned user may separately
receive a feedback of an SINR and/or the additionally assigned user may use AMC which
always shows a low rate.
[0042] In the third method, the number L of pieces of data to be transmitted is adaptively
determined so that the users can perform feedback for all precoding vectors. In this
case, the BS may adaptively select the number L according to average channel information
and the number of users, and may schedule the users. Further, the BS may regulate
the number L according to a scheduling result, and such a procedure may be repeated
until all precoding vectors are scheduled.
[0043] According to these methods, each BS can assign the users to the 2L precoding vectors
even if a set of precoding vectors fed back is insufficient. However, in the first
method described above, the number of precoding vectors assigned to the users is less
than 2L.
[0044] According to the embodiment of the present invention, each user may feed back information
to all BSs participating in cooperative communication or may feed back information
only to some BSs. Whether a user will feed back information to only some BSs or to
all BSs may be determined according to a setting of a wireless communication system,
but the present invention is not limited thereto. For example, information may be
fed back only to some BSs since a channel condition deteriorates.
[0045] For BS cooperative communication according to the embodiment of the present invention,
information exchange is necessary between BSs if feedback is achieved from or to only
some BSs. In an example described below, a user performs feedback only for one BS
when two BSs participate in cooperation communication.
[0046] When the user performs feedback only for one BS according to a feedback mechanism
determined by the wireless communication system, each BS selects an SINR for maximizing
a total data rate with respect to 2L precoding vectors for each precoding matrix (however,
the aforementioned three methods may be used when one precoding matrix cannot be configured
because the number of users is small, and optionally, the number of selected SINRs
may be less than 2L). In this case, each BS exchanges the selected 2L SINRs to finally
select 2L SINRs for maximizing a total data rate, and then assigns users to the selected
SINRs.
[0047] FIG. 3 is a diagram showing a user scheduling scheme through information exchange
in a two-cell BS cooperative communication system. The two-cell BS cooperative communication
system may be a system in which each user performs feedback only for one BS. Referring
to FIG. 3, a BS 1 and a BS 2 may exchange information for all precoding matrices or
may exchange information only for one precoding matrix. In addition, each BS (i.e.,
the BS 1 and the BS 2) performs user scheduling by selecting 2L SINRs for maximizing
a data rate with respect to each precoding matrix.
[0048] As such, in the user scheduling through information exchange as shown in FIG. 3,
the information exchange may be performed on all precoding matrices or may be performed
on only one precoding matrix. If information obtained by each BS through feedback
cannot configure one precoding matrix, 2L or less number of pieces of information
may be exchanged by using one of the aforementioned methods. Hereinafter, a case of
exchanging 2L pieces of information will be described.
[0049] In the former case, BSs exchange 2L pieces of SINR information for all precoding
matrices. Eventually, the number of pieces of exchanged SINR information is G?2L.
Each BS determines a precoding matrix for maximizing a total data rate by using the
exchanged information together.
[0050] In the latter case, a value of the total data rate and an index of the precoding
matrix may be first exchanged, and then 2L pieces of SINR information may be exchanged.
For example, in the latter case, an operation may be performed according to a method
of FIG. 4. First, for a precoding matrix for maximizing a total data rate, each BS
exchanges an index of the precoding matrix and information on the total data rate
(step S21). Then, each BS determines whether exchanged precoding matrices have the
same index (step S22). If the determination result shows that the exchanged precoding
matrices have different indices, 2L pieces of SINR information are exchanged for a
precoding matrix selected by a BS having a higher total data rate, and then users
are scheduled to maximize the total data rate (step S23). On the other hand, if the
exchanged precoding matrices have the same index, the 2L pieces of SINR information
are exchanged for the precoding matrices, and then the users are scheduled to maximize
the total data rate (step S24).
[0051] In the embodiment of the present invention described above, two-cell cooperative
wireless communication system has been described. However, as described above, the
embodiment of the present invention can also be applied to a two or more-cell cooperative
wireless communication system, which will be described below.
[0052] If the number of BSs participating in cooperative communication is greater than two
cells, a multi-cell precoding matrix has to be reconfigured. Equation 9 shows a multi-cell
precoding matrix which can be configured when 3 BSs participate in cooperation communication.
[0053] Herein, W
p, W
q, and W
r denote single-cell precoding matrices, and a
mn is any possible combination for satisfying
(herein, W
coH denotes a Hermitian matrix of W
co, m=1, 2, 3, and n=1, 2, 3). In addition, p, q, and r may be all identical values,
or some or all of them may have different values.
[0054] If the number of BSs participating in cooperation is C, the multi-cell precoding
matrix may be as expressed by Equation 10. Equation 11 having a form of a discrete
Fourier transform (DFT) matrix may be used for a
mn of Equation 10. By using Equation 10 and Equation 11, the multi-cell precoding matrix
can be configured even if the number of BSs participating in cooperation is greater
than or equal to 3. In Equation 10, W
p, W
q, ..., W
r each denotes a single-cell precoding matrix, a
mn may be any combination for satisfying
(herein, W
coH is a Hermitian matrix of W
co, m=1, 2, ... ,C, and n=1, 2, ..., C). In addition, p, q, ..., r may have the same
value, or some or all of them may be different values.
[0055] If the number of BSs participating in cooperation increases, information exchange
performed between BSs has to be performed between all BSs. In this case, each BS may
exchange information in a one-to-one manner, or necessary information may be shared
by the use of an additional storage or a central server for information exchange.
In the latter case, each BS has to transmit all or some information obtained through
feedback to the central server.
[0056] Although a scheduler for maximizing a data rate has been described above for example
in the aforementioned embodiments, the embodiments of the present invention are not
limited thereto. For example, other types of schedulers (e.g., a pro- portional fair
scheduler) may also be used. In this case, SINR information obtained through feedback
and/or information exchange may be changed into matrix in- formation in a form usable
by a corresponding scheduler, and thus users can be assigned in the same manner.
[0057] FIG. 5 is a block diagram showing a wireless communication system to implement an
embodiment of the present invention. A BS 50 may include a processor 51, a memory
52 and a radio frequency (RF) unit 53. The processor 51 may be configured to implement
proposed functions, procedures and/or methods described in this de- scription. Layers
of the radio interface protocol may be implemented in the processor 51. The memory
52 is operatively coupled with the processor 51 and stores a variety of information
to operate the processor 51. The RF unit 53 is operatively coupled with the processor
11, and transmits and/or receives a radio signal. A UE 60 may include a processor
61, a memory 62 and a RF unit 63. The processor 61 may be configured to implement
proposed functions, procedures and/or methods described in this de- scription. The
memory 62 is operatively coupled with the processor 61 and stores a variety of information
to operate the processor 61. The RF unit 63 is operatively coupled with the processor
61, and transmits and/or receives a radio signal.
[0058] The processors 51, 61 may include application-specific integrated circuit (ASIC),
other chipset, logic circuit and/or data processing device. The memories 52, 62 may
include read-only memory (ROM), random access memory (RAM), flash memory, memory card,
storage medium and/or other storage device. The RF units 53, 63 may include baseband
circuitry to process radio frequency signals. When the embodiments are implemented
in software, the techniques described herein can be implemented with modules (e.g.,
procedures, functions, and so on) that perform the functions described herein. The
modules can be stored in memories 52, 62 and executed by processors 51, 61. The memories
52, 62 can be implemented within the processors 51, 61 or external to the processors
51, 61 in which case those can be communicatively coupled to the processors 51, 61
via various means as is known in the art.
[0059] The preferred embodiments of the present invention have been described with reference
to the accompanying drawings, and it will be apparent to those skilled in the art
that various modifications and variations can be made in the present invention without
departing from the scope of the invention. Thus, it is intended that any future modifications
of the embodiments of the present invention will come within the scope of the appended
claims and their equivalents.